Using flow cytometry, we studied the expression of the CD4 antigen within the different cells present in human ejaculate, both in spermatozoa and round cells. In all, 20 samples of semen were obtained from fertile males; in 11 of these, we detected the presence of leukocytes, using the peroxidase test. Swim-up was performed for the analysis of the spermatozoa. From our results it may be concluded that there is no expression of the CD4 antigen on the surface of human spermatozoa or on CD45- ejaculate cells (epithelial and germinal cells). However, we did detect the presence of the CD4 antigen on the surface of the leukocyte cells (CD45+). A better characterization of these CD45+ cells made it apparent that the CD4+ cells of ejaculate are composed of T lymphocytes (helper/inducer T lymphocytes) and monocytes. Thus we may conclude that human spermatozoa do not express the CD4 antigen, the cell surface receptor for human immunodeficiency virus. However, we did detect CD4+ T lymphocytes and CD4+ monocytes in semen.